I have a 1965 Lincoln Continental that will not shift out of 1st gear while driving. I recently replaced the intake manifold gaskets due to a coolant leak. Before I did this, I didn’t have any issues with the transmission. Now there is no coolant leak at the intake manifold, but I have checked for a vacuum leak and the gauge shows 13-14 inches of mercury at idle. The manual says that it should be 18 inches of mercury minimal.
I have a few questions. First, is there any way to any way to correct the vacuum leak at the manifold without taking it back off? Second, would a low vacuum cause the issue with the transmission not shifting into higher gears?
It is difficult to answer your first question regarding the vacuum leak because you don’t say where at the manifold the leak actually is. If the leak appeared only after you replaced the gaskets though I would say yes it would need to be removed and checked for a gasket out of place or the possibility that the wrong gaskets were supplied to you. If you rotated the distributor during the manifold gasket replacement the resultant incorrect ignition timing can affect the vacuum gauge reading. If a vacuum fitting on the manifold was damaged or not reinstalled etc. etc. that alone could be the only cause of your low vacuum reading. The transmission does require vacuum in order to shift properly and in fact has its own vacuum supply from a fitting at or near the intake area to a valve known as the “Modulator Valve” at the rt. rr. of the transmission. This possible lack of vacuum and the possibility of the transmission kick down rod from the carburetor linkage to the transmission linkage being out of adjustment or not reinstalled will result in poor transmission operation. To sum up, a simple vacuum disconnect may be your only problem so I would advise you to diagnose very carefully before deciding to pull the manifold again. Good luck with a simple fix.